The interaction of different sensory systems (vision touch, hearing, etc.) and their mutual substitution are hallmarks of real-world perception. Using haptic and tactile sensations, humans can accurately perceive and usefully employ hand-held objects without vision. Previous research has indicated that naive, blindfolded subjects can use a hand- held probe for a simple mobility task, the crossing of a gap in the path of motion. However, the sensory basis of this ability is obscure. Haptic and tactile perception are clearly involved, but qualities of a hand-held object (such as length and mass distribution) that ordinarily affect haptic perceptions of surfaces contacted with that object, do not disrupt performance, and existing investigations of the utility of the sounds of probe-surface contact for similar tasks are inconclusive. The proposed project would test the candidate modalities using the gap-crossing paradigm, in which blindfolded subjects explore gaps in a raised path with a probe, and cross the gap or judge its crossability. For generality, the performance of skilled blind individuals at this task will be analyzed and compared with extant data on naive blindfolded subjects. This information will establish a base for rigorously investigating the role of the sound of contact; acoustic analyses will isolate the information available in this situation. Experiments on the kinesthetic information in this situation will be conducted. Preliminary results, as well as general understanding of the overdetermined nature of perception of practical aspects of the environment, suggest that none of the modalities may be indispensable, but that observers may be able to compensate for a loss or disruption of any single category of information. Evidence of this kind of compensation will be sought in the subject's style of exploration.